The present invention relates generally to the use of a sintered granular metal as part of a porous filter element for use within a filter cartridge or as a separate filtering device. More specifically the present invention relates to the use of a non-restrictive structural support member as part of a porous filter element which is formed by using a sintered granular metal.
It is generally known that sintered powder metal can be used as a filtering element for mechanically separating suspended matter from a liquid or gaseous fluid medium. There are a number of different materials which may be used and a variety of design options for the filter assembly. One such design is disclosed in U.S. Pat. No. 3,984,044 which issued Oct. 5, 1976 to Breton, et al. The Breton et al. patent describes a filter structure which comprises at least two laminated, porous, plate-like filter structures having relatively large planar front and rear surfaces joined by relatively small edge surfaces arranged substantially parallel to each other and spaced up to approximately five, and preferably less than one, inches apart. Each filter structure comprises at least two contacting and adhered, substantially coextensive porous layers whose pores are in open communication over substantially all of the interface between their contiguous surfaces so as to permit flow of the fluid medium from one layer to the other. One of the layers is a retention layer for the suspended matter having a pore size sufficiently small substantially to prevent passage of the suspended matter to be removed from the fluid medium and the other of the layers is a supporting layer having a pore size larger than the pore size of the retention layer.
The Breton, et al. patent states that the object of the described mechanical separation, specifically clarification by filtration, is for clarifying hot, viscous, liquid suspensions such as the clarification of polymer melts and polymer solutions as is required immediately preceding an extrusion operation. The claimed novelty of the Breton, et al. invention is said to reside in a series of plate-like filter structures which provide a high ratio of retention surface area per unit volume of filter element, this ratio ranging from between 4 and 60 or 70.
Other porous filter element designs are disclosed by U.S. Pat. No. 5,202,022 which issued Apr. 13, 1993 to Ferri and U.S. Pat. No. 4,579,658 which issued April 1, 1986 to Moller. The Ferri patent discloses a porous filter support plate of the kind used in traveling bridge filters for the support of granular filter media which is used in the filtration of waste water, industrial liquors or baths, or for filtration of potable water in a water purification system. The support plates are formed of porous, heat-fusable materials, for example, a thermoplastic organic material, joined together by heat, fused, butt joints and/or reinforced by vertical zones which extend vertically through the plates in which the material has been brought to a molten state and is pressed together to form a dense, solid, non-porous mass.
The Moller patent discloses a sheet-like filter element which includes a porous or fibrous filtering layer and a pair of opposite cover layers. The cover layers are maintained in tight engagement with the outer surfaces of the filtering layer by means of transversely extending connecting strings. The filter element may, for example, be used as a filter for a gas venting opening in a stoma bag, and in that case the porous filtering layer contains a large amount of finely granulated activated carbon. The connecting strings maintain the cover layers pressed against the filtering layers so as to avoid any tendency of delamination of the filtering layer and formation of undesired spaces or passages therein.
The use of sintered granular metals (powder metal), such as stainless steel, steel, copper/nickel alloys, bronze and brass, for filter elements can provide certain advantages. For example, these filter elements offer the advantages of high efficiency, rigid pores and filter elements which are non-migrating. There is though one limitation to the present state in the art in sintered granular metal filter elements. These filter elements have a relatively low resistance to high differential pressures such as those pressures present in high pressure fluid systems. If the wall thickness of the sintered granular metal filter element is increased so as to resist the high differential pressures, then the resulting pressure losses through the filter element will be prohibitive.
In order to address the primary limitation of sintered granular metal filter elements and solve this problem in a novel and unobvious manner, the present invention was conceived. The present invention provides a high collapse pressure, porous filter element (and device) which combines the desired (correct) filtration performance with higher structural strength in a unitized filter element. This preferred combination of filter element features is achieved by placing a non-restrictive structural support member in the granular metal (power metal) prior to the compacting step in the forming die. When the "green compact" is produced, it has the size and shape of the finished filter element with the structural support member embedded therein. The amount (volume) of granular metal which is used for the filter element relative to the size of the compacting die and the individual granule size controls the pore size and density of the filter element.
During the sintering step, the oven temperature reaches a level which is at or just below the melting temperature of the granular material. When the temperature is held at this level, the contact points between adjacent and abutting metal granules are fused or welded together. The result is a rigid filter element with fixed, rigid tortuous pores (passages). The structural support member is locked into the metal granules and is similarly fused or welded therewith so as to create the desired filter element as a single, unitized component.